Diurnal preference is highly variable in the population and influenced by numerous factors. Extreme diurnal types, however, are relatively rare and exhibit trait-like differences in both sleep timing and circadian pacemaker function. Studies in moderate morning and evening types have revealed alterations of habitual sleep timing, circadian phase and period of temperature and hormonal rhythms. Genes regulating circadian function have been characterized in multiple species. Two human circadian genes have independently been associated with sleep timing in humans, suggesting considerable overlap between circadian and sleep function at the genetic and molecular levels. The proposed study will determine the genetic basis of extreme diurnal preference phenotypes. Candidate circadian genes will be directly sequenced in DNA from definite morning and evening types carefully screened and selected and physiologically characterized for sleep and circadian function. Studies of the genetic basis of extreme diurnal preference represent a new and exciting avenue to elucidate how the sleep-wake and circadian systems are integrated with each other, and how alterations in specific circadian clock genes can lead to alterations in the timing of sleep and wake.